An assembly battery (battery system) configured by connecting, in series or in series-parallel, a plurality of cell groups each including a plurality of battery cells of secondary battery in series is used for a hybrid electric vehicle (HEV), an electric vehicle (EV), or the like to secure a desired high voltage. In such an assembly battery, a cell controller which measures a cell voltage (the voltage across the terminals of a battery cell) and performs balancing discharge for equalizing (balancing) the states of charge (SOC, State of Charge), that is, the remaining capacities is provided in a monitoring apparatus of the assembly battery to calculate the remaining capacity of each battery cell and protect and control the battery cells, and accordingly the assembly battery is controlled (for example, see PTL 1). Moreover, the cell controller includes a plurality of integrated circuits (cell controller ICs) and controls the plurality of cell groups.
The SOC of each battery cell is calculated from the open circuit voltage (OCV) of the battery cell. In other words, the SOC of each battery cell is calculated from the voltage across the terminals of the battery cell measured in a state where the battery system is under no load conditions, in other words, in a state where the battery system is not connected to things such as an inverter which generates three phase AC power from DC power from the battery system to supply the three phase AC power to a drive motor of HEV or EV. The voltage across the terminals of each battery cell is measured by being input into a voltage input terminal of the cell controller IC. The voltage input terminal of the cell controller IC is provided with an RC filter to remove noise accompanied with the charge/discharge of the battery system.